The present invention relates to an optical reading apparatus for a facsimile system or the like.
In a typical optical reader an optical system scans an original document such as a page of a book and focusses a light image of the document onto a photosensor array which produces electrical data signals corresponding to the light image. These signals are stored in a buffer memory. In order to reduce transmission time in a facsimile system, the data signals read from the buffer memory are compressed in such a manner as to reduce redundant data corresponding to blank background areas of the document and the like. The data compression rate varies in accordance with the amount of redundant data. More specifically, the higher the proportion of high density areas of the document corresponding to printed characters and the like (the lower the proportion of redundant white background areas) the lower the data compression rate.
The optical system generally comprises a transparent platen which supports the document and is moved by a drive motor for vertical scanning. The photosensor array typically comprises a linear array of photosensor elements which are arranged perpendicular to the direction of movement of the platen. The elements are sequentially enabled or strobed for horizontal scan.
If the platen were moved at constant speed, a buffer memory having a capacity large enough to accomodate an entire document page would have to be provided to compensate for the variation in the data compression rate. Such a memory would be extremely expensive and large in size, even utilizing integrated circuit technology.
A system has been proposed in the prior art which comprises a pulse or stepping motor to move the platen for scanning and a buffer memory having a relatively small capacity. The stepping motor is energized in accordance with the data compression rate to intermittently effect vertical scan as the data in the buffer memory decreases from a predetermined median value. Such a system is responsive to intricate patterns on the document. However, the stepping motor must be of disproportionately high power to intermittently move a heavy glass platen which supports a heavy document such as a book. Such a motor is large, expensive, and wasteful of electrical energy.